1. Field of the Invention
This invention relates to methods and apparatus for protecting electrical circuitry from reverse engineering, and more particularly to the protection of codes stored in a memory circuit.
2. Description of the Related Art
It is very important to prevent unauthorized access to certain portions of electrical circuits, particularly integrated circuits. For example, digital codes or other data may be stored in certain portions of the circuit to prevent its unauthorized use; the circuit will function properly only if the user enters the appropriate code. Such circuitry may be compromised, however, by various methods of analysis, such as visual inspection, microprobing, x-ray, secondary electron emission voltage-contrast analysis, etc. A number of techniques have been used in the past to prevent such reverse engineering.
Perhaps the most basic technique is to enclose the circuit in a plastic encapsulation or protective die coating. However, access to the circuitry can still be gained by mechanical or acid drilling through the encapsulation material, or by sawing through the encapsulation. Another approach is to add misleading circuit topology so as to disguise the portion of the circuitry to be protected, or to customize each different die with its own specific code. An example of this approach is disclosed in U.S. Pat. No. 4,766,516, xe2x80x9cMethod and Apparatus for Securing Integrated Circuits from Unauthorized Copying and Use,xe2x80x9d Aug. 23, 1988, to Ozdemir et al. and assigned to Hughes Aircraft Company. These techniques may retard, but generally do not totally prevent, successful reverse engineering.
The present invention seeks to provide a protection system and method that is more reliable than prior techniques for preventing the reverse engineering of secure circuitry, and which operates automatically if the circuit package is tampered with.
The new protection technique provides a mechanism for modifying the secure circuit in case reverse engineering is attempted, so that the original circuit to which access is sought is no longer available. An actuating device for the modification function is provided that responds to radiation of the type used in a reverse engineering analysis, but to which the secure circuit is normally not exposed. When a reverse circuit analysis is attempted, the actuating mechanism senses the radiation and triggers the modification system to alter the secure circuitry.
When the secured circuit is volatile and is maintained by a power supply circuit, the modification system can be used to interrupt the power supply circuit. The interruption may be accomplished by means of a bipolar transistor switch, with a radiation responsive bias circuit for the switch. Other modification techniques include applying an overcurrent to the secure circuit to destroy its functionality, or reprogramming a secure programmable memory.
One actuating mechanism that can be used is an operational amplifier having a pair of differential inputs that are controlled by a radiation responsive element such as a photodiode. To reprogram a secure programmable memory circuit, the actuator may consist of a random access memory (RAM) cell formed from photosensitive transistors. The output from the RAM cell is connected to the secure memory""s address line for reprogramming upon exposure to the radiation. Reprogramming can also be accomplished by an operational amplifier with a radiation responsive input, and an output circuit connected to reprogram the secure memory. A multivibrator connected to the secure memory""s address line through a radiation responsive switch is another reprogramming option.
The protection system can be made responsive to virtually any type of radiation for which a sensitive element is available. This includes optical radiation (both visible and infrared), x-rays, and an electron beam (which may be considered as a form of radiation) from a scanning electron microscope.